Get yourself a stick of 256MB PC133 CAS2, then sell your 128MB stick. Don't mix them if your other stict is not the same brand. Does it work sometimes, sure, it also doesn't work sometimes and the result is weird crashes, lockups, or other odd things.
I've been doing this too long to ever mix ram, be it type, manafacture, or speed. Generally I buy however much I need, then sell whatever is in the box. Trying to match them is like playing Russian Roulette.
You can saftly run the PC133 at PC100, you can sometimes run PC100 at PC133, but I don't recomend it. Yea, lots of people on this board do it, then lots of people complain that Windows is not stable. Well, my system never crashes (hasn't crashed in as long as I can remember, at least a year or two, why? I buy the best, never mix stuff, and don't overclock. (I'm no fun, am I?) (My system used to crash when I bought on price. When I switched to buying on quality, that stopped, then again I also reinstall every year and mantain my system so that might help as well)
As always, YMMV,
Jason